1. Field of the Invention
This invention relates to an optical recording medium and its production method.
2. Prior Art
Typical optical recording mediums include write once disks such as CD-R and DVD-R wherein an organic dye is used for the recording material, and rewritable disks such as CD-RW and DVD-RW wherein a phase change recording material is used for the recording material. In these optical recording mediums, the recording layer is formed on the disk substrate which has been formed with a tracking groove (continuous concave portion) In the cases of the DVD-R and the DVD-RW which are to be recorded at a high density, the grooves are used for the recording tracks, and in addition, prepits bearing preformat information including the addressing information are formed along the continuous convex portion (which is called land) between the adjacent grooves. Typical embodiments of the medium of such structure are shown in FIGS. 5 and 6. In the medium of such embodiments, a recording layer (not shown) is formed on the substrate 102, and substantially parallel grooves 104 are formed on one surface of the substrate 102. The recording/reading laser beam is irradiated from the lower side of the drawings. The groove 104 is a recording track wherein record marks 10 are formed, and a land 103 is defined between the adjacent two grooves 104 and 104. The land 103 is formed with prepits 105. The optical disk which has the structure shown in FIG. 5 is disclosed in Japanese Patent Application Laid-Open No. (JP-A) 326138/1997, FIG. 15, and the optical disk with the structure of FIG. 6 is shown in JP-A 326138/1997, FIG. 3 and JP-A 132868/2000, FIG. 1(a).
Next, a typical process for producing a disk substrate is described.
A disk substrate is typically prepared by injection molding a resin using a stamper having formed thereon a mother pattern for the prepits and grooves. The stamper is usually formed from nickel. In order to manufacture the stamper, a disk master is first prepared, and a master stamper (a disk after one transcription), a mother stamper (a disk after two transcriptions), a child stamper (a disk after three transcriptions), and the like are prepared from the diskmaster.
The following process is generally employed for the manufacture of the disk master. First, a resist layer comprising a resist material such as photoresist is applied on the surface of a rigid substrate comprising a material like glass. The resist layer is then exposed to an exposure beam such as laser beam to form a latent image of the desired pattern, followed by development. The disk master is thereby produced by the patterning of the resist layer.
Using the disk master, a stamper is manufactured usually by the following process. First, to impart electric conductivity to the surface of the resist layer on the disk master, a nickel thin film is formed as by sputtering or electroless plating. Electroforming is then effected to deposit an electroformed nickel film on the nickel thin film. Then, the laminate of the nickel thin film and the electroformed nickel film is stripped from the resist layer. The laminate is ready for use as the master stamper. The mother stamper is prepared by electroforming a film on the surface of the master stamper and striping the electroformed nickel film. It is recommended to previously oxidize the surface of the master stamper so that the electroformed nickel film may be readily stripped therefrom. Alternatively, a child stamper may be similarly prepared using the mother stamper and used as the stamper.
In the process of preparing a disk master, the minimum width of the latent image pattern formed in the resist layer is limited by the diameter of a laser beam spot. The beam spot diameter w is represented by w=kxc2x7xcex/NA wherein xcex is the wavelength of the laser beam, NA is the numerical aperture of an objective lens in an optical system, and k is a constant which is determined by the aperture shape of the objective lens and the intensity distribution of an incident light flux. In the formation of a latent image pattern corresponding to the groove, the beam is scan in spiral manner. When prepits are to be formed between the grooves, two beams are used in the formation of the latent image pattern, and one beam is continuously irradiated for the formation of the groove pattern while the other beam is intermittently irradiated to form the prepit pattern. The process as described above wherein two beams are used is referred in the present invention as xe2x80x9ctwo beam processxe2x80x9d.
As described above, in the manufacture of a disk master of the structure wherein prepits are formed between the grooves, independent control of the two exposure beams is required in the photolithography, and structure and control of the exposure system inevitably becomes complicated.
In addition, improvement in the recording density requires decrease in the length of the prepits. The conventional process wherein the resist layer is intermittently irradiated with the beam and the area irradiated with the beam is used for the prepit is associated with the problem that the size of the prepit is limited by the wavelength of the beam as well as the numerical aperture of the optical system used for the irradiation. To be more specific, when the length of the prepit is reduced (that is, when the exposure time is reduced) with the wavelength of the beam and the numerical aperture kept unchanged, the resist layer will not be dug to a depth near its presumed depth in the formation of the prepit pattern in the resist layer, and this results in the failure of forming the prepits having the predetermined depth, and hence, in the difficulty in reading the prepits. In the meanwhile, substantial change in the exposure system such as decrease in the wavelength of the beam used or change of the objective lens to the one having a larger numerical aperture invites significant increase in the cost.
An object of the present invention is to facilitate the production of the supporting substrate and to facilitate a high density recording in an optical recording medium wherein grooves or lands formed in the supporting substrate are used for the recording tracks and wherein prepits are formed.
Such objects are attained by the present invention as described in (1) to (12), below.
(1) An optical recording medium comprising a supporting substrate and a recording layer on the substrate wherein the surface of the supporting substrate on which the recording layer is formed is formed with alternating continuous convex portions and continuous concave portions which are substantially parallel to each other, wherein
the adjacent continuous convex portions are connected by bridging convex portions which extend in transverse direction, and the bridging convex portion has a planar configuration that its width is minimum at or near the center between the adjacent continuous convex portions.
(2) An optical recording medium according to the above (1) wherein said bridging convex portions are used as prepits.
(3) An optical recording medium according to the above (1) or (2) wherein isolated concave portions are formed in said continuous convex portions and said isolated concave portions are used as prepits.
(4) An optical recording medium according to any one of the above (1) to (3) wherein the continuous concave portion has winding parts that protrude into the adjacent continuous convex portion, and the protruding parts formed in the adjacent continuous convex portion are used as prepits.
(5) An optical recording medium comprising a supporting substrate and a recording layer on the substrate wherein the surface of the supporting substrate on which the recording layer is formed is formed with alternating continuous convex portions and continuous concave portions which are substantially parallel to each other, wherein
the adjacent continuous concave portions are connected by bridging concave portions which extend in transverse direction, and the bridging concave portion has a planar configuration that its width is minimum at or near the center between the adjacent continuous concave portions.
(6) An optical recording medium according to the above (5) wherein said bridging concave portions are used as prepits.
(7) An optical recording medium according to the above (5) or (6) wherein isolated convex portions are formed in said continuous concave portions and said isolated convex portions are used as prepits.
(8) An optical recording medium according to any one of the above (5) to (7) wherein the continuous convex portion has winding parts that protrude into the adjacent continuous concave portion, and the protruding parts formed in the adjacent continuous concave portion are used as prepits.
(9) An optical recording medium according to any one of the above (1) to (8) wherein said continuous convex portion or said continuous concave portion is used for the recording track.
(10) An optical recording medium according to any one of the above (1) to (9) further comprising a substrate or layer which is transparent to the recording/reading laser beam formed on said supporting substrate, wherein said recording layer is irradiated by the laser beam that has passed through said substrate or layer, wherein, said continuous convex portions function as grooves and said continuous concave portions function as lands.
(11) An optical recording medium according to any one of the above (1) to (9) wherein said supporting substrate is transparent to the recording/reading laser beam and the recording layer is irradiated by the laser beam that has passed through the supporting substrate, wherein
said continuous convex portions function as lands and said continuous concave portions function as grooves.
(12) A method for producing an optical recording medium comprising a supporting substrate and a recording layer on the substrate wherein the surface of the supporting substrate on which the recording layer is formed is formed with alternating continuous convex portions and continuous concave portions which are substantially parallel to each other, and prepits are formed along the continuous convex portions or the continuous concave portions, wherein
the disk master used for the production of said supporting substrate having a resist layer formed with a mother pattern of said continuous convex portions, said continuous concave portions, and said prepits is formed by
irradiating the resist layer with an exposure beam to form the mother pattern, and the step of the exposure is accomplished by intermittently irradiating the layer with one exposure beam and using the area irradiated with the exposure beam for the mother pattern of the continuous concave portion or the continuous convex portion, and the area that was not irradiated with the exposure beam for the prepits.